Phase-locked loops (PLLs) are used to generate a signal having a particular oscillation frequency. The oscillating signal is often utilized by another electronic system, for example, as the clock signal for a processor. Many PLLs utilize voltage-controlled oscillators (VCOs) to produce the oscillating signal at a particular frequency based on an applied voltage (or control voltage). The PLLs adjust the voltage applied to the VCO until the frequency of the oscillating signal is equal to the desired frequency. In most systems, the VCO is designed to accommodate a range of possible oscillation frequencies over a predetermined input voltage range.
Many VCOs include a number of transistors or other semiconductor devices suitably configured to create an electronic oscillator. Although the VCO may be designed to accommodate a particular range of oscillation frequencies over a predetermined applied voltage range, process, voltage, and temperature (PVT) variations affect the operating parameters for the components that comprise the VCO, which in turn, affect the range of oscillation frequencies achievable by the VCO over the input voltage range. As a result, the range of oscillation frequencies capable of being achieved by the VCO is reduced (e.g., at the high end and/or the low end of the desired frequency range).